Method for continuous welding together of plastic sheets

Abstract

A method and apparatus for continuous welding together of sheets of the type in which adhesion is derived from the melting and re-hardening of the sheet material itself rather than from an externally applied adhesive. A sheet, formed into a tube with the opposite sides overlapping, is tensioned longitudinally so as to form longitudinally extending stress lines which appear as ripples, at least at the overlapping edges such that the sheets weld at the bottom of the valleys formed by the ripples. As the tensioned sheet moves continuously between a heating element and a backing plate, both of which are stationary relative to the tube forming means, the sheets weld as the material from the sides of the valley of at least one sheet runs to the bottom of the valley, at which point the weld bead is formed, whereby the thickness of the weld bead may be more than twice the thickness of the original unheated sheet material.

I claim: 
     
       1. A method of sealing together first and second adjacent overlapping edges of sheet material of the type joinable by welding, comprising: tensioning the sheet material in the direction parallel to said edges to form in at least said first overlapping edge stress lines which form a valley opened upwardly and extending substantially parallel to said edges and such that the ridge on the opposite side of the sheet from the valley protrudes downwardly towards and touches the second sheet, and applying heat to said overlapping edges to melt the said first overlapping edge to weld the overlapping edges in the vicinity of said valley by causing the melted sheet material of the first overlapping edge to flow down the sides of the valley to thicken the overlapping edges at the bottom of the valley at the weld. 
     
     
       2. A method according to claim 1, wherein both said overlapping edges are tensioned, and the valleys of the overlapping edges touch each other. 
     
     
       3. A method according to claim 1, said sheet material being a sheet formed into a tube, the opposite sheet edges constituting said overlapping edges, and wherein the steps of tensioning comprises tensioning the sheet in said direction of the edges across the entire width thereof. 
     
     
       4. A method according to claim 3, wherein the heat is applied only to the surface of the overlapping edges adjacent the first overlapping edge and including supporting the opposite side of the overlapping edges by a backing surface which is stationary relative to the forming means. 
     
     
       5. A method according to claim 1, wherein the amount of material which flows is dependent on the heating temperatures, and wherein said heating step comprises heating the overlapping edges to a temperature such that sufficient material flows to form at the weld a thickness more than the total original unheated thickness of the two overlapping edges. 
     
     
       6. A method according to claim 1, wherein said sheet material is polyethylene. 
     
     
       7. A method according to claim 1, wherein the heating step comprises heating the overlapping edges with a heating element on the side of the overlapping edges adjacent the said first overlapping edge, and including supporting the opposite side of the overlapping edge with a supporting means stationary axially relative to the heating element. 
     
     
       8. A method according to claim 7, including moving the overlapping edges continuously axially relative to the heating element and the supporting means. 
     
     
       9. A method according to claim 8, including applying sufficient heat to form at the weld sheet material having a thickness more than twice the thickness of the original unheated sheet material. 
     
     
       10. A method according to claim 7, said step of heating comprising heating with radiant energy without the application of pressure to the overlapping edges. 
     
     
       11. A method according to claim 1, wherein the heat is applied on the surface of the overlapping edges at which the said one overlapping edge with the valley is located, and including supporting the opposite surface of the overlapping edges with a support means which is stationary relative to the forming means. 
     
     
       12. A method of coating a pipe with a sheet material comprising the following steps: with a tube forming means, forming a length of sheet material into a tube of coating material surrounding the pipe to be coated, such that the opposite edges of the sheet overlap circumferentially on top of the tube,   establishing relative axial movement between the pipe and the tube forming means such that additional pipe length is presented to the forming means to receive newly formed lengths of the tube of coating material,   tensioning said tube of coating material in the said axial direction by a sufficient amount to form in at least the upper one of said overlapping edges stress lines which form a valley open upwardly and extending parallel to the said axis,   and heating the overlapping edges to heat at least the said upper overlapping edge to the melting point of the sheet material, and welding the overlapping edges together in the vicinity of the said valley by causing the melted sheet material of the first overlapping edge to flow down the sides of the valley to thicken the overlapping edges at the bottom of the valley at the weld.   
     
     
       13. A method according to claim 12, including delivering the sheet material from a supply roll to said forming means, and wherein the step of tensioning includes applying tension to the sheet material between said supply roll and said forming means. 
     
     
       14. A method according to claim 13, wherein said tension is applied across the entire width of the sheet such that said valleys are formed in both of said overlapping edges. 
     
     
       15. A method according to claim 12, wherein the amount of material which flows is dependent on the heating temperature and wherein said heating step comprises heating the overlapping edges to a temperature such that a sufficient amount of the sheet material flows to form at the weld a thickness more than the total original thickness of the two overlapping edges. 
     
     
       16. A method according to claim 12, wherein said sheet material is polyethylene. 
     
     
       17. A method according to claim 12, including providing first and second of said tubes concentrically on a pipe, with an annular space therebetween, shrinking said first tube to bring said first tube into close adjacency with the outer surface of said pipe and then providing said second tube therearound, introducing a settable, expandable, filler composition into the annular space between said first and second tubes to form a tubular layer of settable filler between said tubes, permitting said settable filler composition to expand and set to form a set, expanded, annular tubular layer of filler composition around said first tube and bringing the second tube into close adjacency with said tubular layer of filler composition thereby providing a pipe encased in a multi-layer protective wrapping.